1. Field of the Invention
The present invention relates to a discharge recording medium having a colored layer, a metal-vaporized layer and, as required, a protection layer formed in that order on a support member composed of, for example, a paper or plastic film.
2. Description of the Prior Art
In recent years, a so-called discharge recording medium has been used extensively to record characters, symbols, figures and the like in response to electric data signals.
According to the art of discharge recording employing such a discharge recording medium, a signal voltage is applied across a recording stylus and an earth electrode (ground electrode) on the surface of discharge recording medium which has a colored layer (lower layer) and a metal-vaporized layer (upper layer) formed on a support member, whereby the metal-vaporized layer beneath the recording stylus is destroyed by the electric discharge, so that the lower colored layer is exposed and made visible.
The colored layer of the discharge recording member consists of a coloring agent such as carbon black or dye-stuff and a bonding resin, and has a great surface resistance, i.e., has electrically insulating property.
When a recording voltage is applied to the above-mentioned conventional recording material, however, a concentrated discharge takes place in a portion of the recording layer, a heavy current flows instantaneously, and the recording layer is destroyed, giving rise to the occurrence of the following undesirable phenomena:
(i) Picture lines and dots are coarsened with the result that the picture becomes obscure. PA1 (ii) Contaminants are generated in large amounts by the discharge, and fused (deposited) onto the recording stylus. Therefore, the recording stylus loses its electrical conductivity, and it becomes difficult to continue recording. In extreme cases, it becomes virtually impossible to carry out recording. PA1 (iii) The metal-vaporized layer is destroyed in the place where the earth electrode is located (i.e., earth discharge), and the picture becomes obscure.
In recent years, in particular, the discharge recording apparatus of this type can be represented by a so-called multistylus-type apparatus employing a head which incorporates 5 to 200 recording styluses.
With this apparatus, however, a signal voltage must be simultaneously applied to a plurality of recording styluses. Therefore, the discharge current as a whole becomes very great, and the above-mentioned defects of the conventional recording media tend to appear even more strikingly.
In order to prevent the above-mentioned defects, therefore, there has been proposed a method of increasing the amount of carbon black in the colored layer in an attempt to make the colored layer electrically insulative (surface resistance is 10.sup.8 ohms or smaller, see Japanese Patent Laid-Open No. 51-101544, Japanese Patent Publication No. 54-15415).
The above method makes it possible to suppress the occurrence of the above-mentioned phenomena (ii) and (iii), but invite the following new defects.
(1) Since the colored layer is electrically insulative, the discharge current density is reduced when the recording is being made. Therefore, the discharge energy is insufficient, lines and dots correspondingly become too narrow, and discharge becomes defective. This, of course, also means that the picture becomes obscure.
(2) The vaporized metal and the electrically conductive carbon contained in the colored layer form a battery. Therefore, electrolytic corrosion takes place, and the surface is blackened (preserving property is lost).
(3) Carbon black contained in large amounts in the colored layer flies off when the record is being made, and pollutes the recording medium, recording apparatus, and the surrounding environment.
Under such circumstances, the inventors of the present invention have conducted a keen study in order to overcome various defects such as earth discharge, melt-adhesion (deposition), and the like, without making the colored layer electrically insulative. The inventors have found that when a graphite powder is contained in the colored layer, it is possible to improve the discharge characteristics when the recording is being made, to prevent the earth discharge, melt-adhesion (deposition) phenomena, to obtain sharp dots and lines, to eliminate the appearance of scanning lines, to obtain a vivid record and, hence, to obtain a discharge recording medium having excellent preservability, and have thus accomplished the present invention.